The present invention relates to an optical communication module that is applied to a wavelength division multiplexing optical communication system. That is, the invention provides a stable optical system for locking a lasing wavelength of light from a laser source and a control system for the optical system. This optical system can be separately operated as a wavelength locker module, but it also can be integrated into an optical communication module having a laser source.
Optical fiber communication features a long transmission length, high speed and large capacity, and a strong immunity to electromagnetic noises; and, hence, a communication system that assures high reliability can be provided. Formerly, in such systems, light of a single wavelength was transmitted on a single strand of optical fiber. However, with the advent of large capacity computerization in recent years, there has been a strong demand for the transmission capacity to be further increased. Therefore, a wavelength division multiplexing optical communication system has been developed and put into practical use, in which a plurality of optical signals each having a different wavelength are transmitted over a single strand of optical fiber, so that the number of communication channels is increased to achieve a system having a larger capacity. Normally, for the wavelength of light to be transmitted in an optical fiber, use is made of wavelength bands where the transmission loss of the optical fiber is low, and such wavelength bands in a 1.3 μm range and in a 1.5 μm range are called windows of transmission. Since the wavelength widths of these windows are limited, the narrower the wavelength spacing between adjacent channels becomes, the more the number of transmission channels can be increased. Presently, the frequency spacing is set to 200 GHz and 100 GHz, but there is a trend toward further narrowing of the frequency spacing, such as to 50 GHz and 25 GHz. Converting the above-mentioned frequency spacings into wavelength spacings, those values become as narrow as approximately 1.6 nm, 0.8 nm, 0.4 nm, and 0.2 nm. When the wavelength spacing is narrowed to such levels, it becomes necessary for the wavelength of the laser source to be controlled to a constant value with pinpoint accuracy. This is because, if the wavelength of the laser source fluctuates to reach as far as the wavelength of the adjacent channel, there occurs crosstalk with the adjacent wavelength channel at the reception side, and, hence, the reliability of information transmission can not be assured. These wavelength (or, frequency) channels are called ITU-T (INTERNATIONAL TELECOMMUNICATION UNION-TELECOMMUNICATION STANDARDIZATION SECTOR) grids and are acknowledged widely as an ITU recommendation.
On the basis of the aforementioned considerations, there have been proposed several methods for controlling the wavelengths of the laser sources for the wavelength division multiplexing of optical communication systems. For example, a method has been devised for locking the wavelength of the laser diode by introducing a dielectric multi-layer filter, a Fabry-Perot etalon, or the like as a wavelength filter and using feedback to control the wavelength on the basis of the operating temperature of the laser diode. Among these wavelength filters, especially the etalon has characteristics such that transmission peaks appear repeatedly in the wavelength according to the number of orders of multi-interference, and therefore, by adjusting the periods of the transmission curve to the ITU-T grids, a single wavelength filter can be used to lock a plurality of wavelength channels. For example, JP-A-79723/1998 discloses a method of locking the wavelength by dividing light which has passed through the etalon into two portions, detecting the two portions using respective photo detectors, and subtracting one signal from the other signal to derive a wavelength deviation signal, which will be used to lock the wavelength.